Biochar and compost equally improve urban soil physical and biological properties and tree growth, with no added benefit in combination.

Soil compaction can be a major impediment to tree growth as it damages soil physical and biological properties and reduces plant available water. This may result in trees that are more vulnerable to seasonal water stress. Improving soil physical and biological properties by increasing soil organic matter content may lead to improved tree establishment. Organic matter (OM), in the form of municipal green waste compost (MGWC) or biochar was incorporated into compacted urban soils at two sites. We established six soil treatments: 1) unamended, 2) tillage only, 3) tillage with MGWC (20% v/v), 4) tillage with biochar (10% v/v), 5) tillage with MGWC + biochar (10% & 5% v/v - low), and 6) tillage with MGWC + biochar (20% & 10% v/v - high) (one site only). The treatments were established to a depth of 0.5 m in 2 × 2 m plots. One Corymbia maculata sapling was planted into each plot. Bulk density, hydraulic conductivity, stem diameter growth and tree water status were measured during tree establishment. At the end of the 30-month experiment, development of water stable aggregates, the rate of microbiological decomposition of OM, and tree size (diameter at breast height; DBH, and canopy growth index) were measured. All OM amended treatments improved soil physical and biological properties. There were no significant differences among the OM treatments. At the end of the experiment, tree DBH and canopy growth index were greater in the OM treatments than tillage only and unamended. As such, we recommend using local and sustainable forms of OM to improve soils and assist tree establishment in challenging sites where soil water is limited, or evapotranspiration demand is high.